Glass 2. Pisces. 



369 



TeleosteanSj Selacliians) ; in others, a transverse membranous bridge, 

 varying in width, divides the opening into two, anterior and posterior 

 nares, of which the anterior may occasionally be drawn out into a 

 narrow tube ; in many Selachians, this bridge is only represented by 

 a flap, arising from one side, and overlying the aperture without being 

 attached to the other. 



In tlie Dipnoi, tte nasal openings are peculiar, in that both lie within the 

 edge of the upper lip. In the Oyolostomes, the two olfactory pits are 

 united to form a deep unpahed tubular sac, the floor of which Ues closely upon 

 the roof of the mouth; in Myxine, it perforates the roof of the mouth, 

 and consists of a tube open at both ends, connecting the oral cavity with the 

 exterior. 



Taste buds, as already mentioned (p. 21), are present in 

 many Fish (Teleostei), not only in the mouth, but scattered over the 

 surface of the body. 



Groups of peculiar sense-organs occur in connection with the 

 skin ; they consist of modified epidermal cells, some of which bear 

 sensory hairs ; and are thus not unlike taste-buds, from which, how- 

 ever, they differ in form. These sensory papillee are supplied 

 with nerves, and may lie free upon the surface of the body {e.g., 

 in most Teleostei), in which case they often bear a cylindrical tube, 

 a cuticular structure, which surrounds and protects the hairs 

 (Fig. 305 r) . In other cases, those portions of the skin to which they 

 belong, have sunk in to form small sacs, opening to the exterior 

 (Ganoids) ; or the sacs have become 

 long tubes filled with mucus, running 

 below the skin, provided at one end 

 with an expansion in which the sensory 

 cells lie, whilst at the other, they 

 open to the surface (on the head in 

 Selachians). Further, similar groups 

 of cells are present in the lateral 

 line which occurs in most Fish. 

 This is a narrow tube (an invagination 

 of the skiti), lying close below the 

 surface, and extending along each 

 side of the body. It usually divides 

 into several branches on the head, 

 one branch running over the summit 

 of the head, another above the eye, 

 a third below the eye — both the 



latter reaching the snout — a fourth along the lower jaw. The 

 lateral line is richly supplied with nerves, and communicates 

 with the exterior by a number of openings; sometimes, indeed 

 it is a partially open groove; in many Teleostei, it runs alono' 

 the side of the body and tail through a series of perforated 



Pig. 305. Sensory papilla of 

 a young Teleostean. r the tube, s cells 

 bearing- hairs.— After F. E. Sohultze. 



B B 



